The present invention relates to sensors, and in particular to sensors such as Hall effect devices that include active offset compensation.
Sensors that include calibration abilities are well known. For example see published German patent application DE 37 86 487 T2, U.S. Pat. Nos. 4,951,236, 4,982,351, and 5,051,937. These sensors are often characterized by their precision, wide application, and relatively low manufacturing cost. To provide the requisite precision, either the transducer device or the associated signal processing must include signal conditioning/compensation logic to calibrate the device and correct for external effects (e.g., temperature). For example, this may be achieved by calibrating and signal-conditioning the transducer device directly, which often requires considerable expense. Alternatively, the appropriately designed signal processing device within the sensor may be signal-conditioned, calibrated, and adapted to the particular application of the sensor.
Signal processing devices are designed as electronic circuits and consequently can be economically manufactured and relatively easily adapted to the particular application. Furthermore, special measures can compensate for the undesirable influences (e.g., temperature) on the signal processing device and suppress/reduce the interferences to the measurement signal (e.g., noise) by the undesirable influences, thus resulting in a higher accuracy sensor device.
However, with such sensors, the signal processing device processes the measurement signal after it has been created by the transducer, and thus may garble the measurement signal, especially with respect to the time structure of the signal. For example, in the case of periodic or quasi-periodic measurement signals and/or in the case of signal evaluation by a threshold comparison, this may lead to inaccuracies with respect to the switch-over point and the phase behavior of the sensor.
Therefore, there is a need for a sensor device that includes error correction circuitry such as offset correction to mitigate the effect of the undesirable influences on the sensor.
Briefly, according to an aspect of the present invention, a sensor includes a transducer that provides a sensed signal to a signal processing device. The signal processing device switches the operating mode as a function of frequency, such that the switching device is operated in the switched operating mode at low frequencies and in the continuous operating mode at high frequencies.
In a preferred embodiment, the transducer is a Hall effect sensing device.
In one embodiment, the signal processing device includes clocked digital circuit sections configured to be clocked by a constant frequency clock signal for detecting low frequencies, and by a signal corresponding to the output signal to detect high frequencies.
The signal processing device may also include a switching device to switch the phase position of the measurement signal. Especially when switching between 0xc2x0 and 180xc2x0, with subsequent averaging offset errors can be largely eliminated.
A preferred modification is that at least one switching device has a hysteresis, which can be adjusted by a control signal. Preferably a control device connected to the signal processing device generates the control signal from the measurement signal. Alternatively or additionally, the signal processing device has an offset which can be adjusted by a separate control signal or by the same control signal from the control device. In this connection, a ramp detector can be connected to the control device and can receive the control signal from it, so as to generate an additional output signal.
The control device may obtain the control signal by averaging the output signal from the evaluation device. Alternatively, the control device may obtain the control signal by evaluating the median of the sensor output signal, by evaluating the minima and maxima of the output signal, by evaluating the pulse-duty ratio of the output signal, and by selecting appropriate values from a look-up table which is stored in a memory.
The measurement signal may be processed by forming averages or by detecting filtered peak values.
Advantageously, the inventive sensor is characterized by high accuracy of the switch-over point and phase and nevertheless by uncomplicated circuit technology. In addition, the inventive sensor can be calibrated without great complication.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.